In many applications there is a need to estimate or measure one or more process quantities which characterize the operation of a turbomachine which can be e.g. a turboblower, a turbocompressor or a pump. Estimates of the process quantities may be needed, for example, for controlling a turboblower, a turbocompressor or a pump so that unstable or oscillatory flow conditions, known as surge and stall, can be avoided with a sufficient safety margin and/or the efficiency can be kept on an acceptable range. For example, in conjunction with a turbocompressor, the operation can be characterized with the following process quantities: the mass flow (kg/s) or the volumetric flow rate (m3/s) generated by the turbocompressor, torque (Nm) directed to the impeller of the turbocompressor, rotational speed (1/s) of the impeller, pressure rise (Pa) generated by the turbocompressor, temperature (K) at the inlet, i.e. at the suction side, of the turbocompressor, temperature (K) at the outlet, i.e. at the output side, of the turbocompressor, and parameters related to properties of the gas being compressed such as the gas constant.
Some process quantities can be measured with practically sufficient accuracy using relatively cost efficient measurement equipment. However, measuring some other process quantities may require complex and rather expensive equipment and/or some process quantities may be even unmeasurable. Measuring, for example, the mass flow or the volumetric flow rate with a sufficient accuracy may be a challenging task. Due to the above-mentioned reasons, there has been a tendency to develop methods in which part of the process quantities are measured and one or more of the remaining process quantities or other characteristic values are calculated on the basis of the measured values and possibly also on the basis of pre-tabulated data.
Publication U.S. Pat. No. 5,508,943 discloses a method for calculating the proximity of the operating point of a turbocompressor to a surge limit using measurements carried out in different places of the compressor-process system. In the disclosed method, it is crucial that the calculation is invariant to suction conditions, especially gas composition. The various assumptions used in the method may in certain situations compromise the accuracy depending on the validity of the assumptions in each case.